ITEM: X0137L

LVM block size and disk reads for raw devices

Question:

Env:    AIX 3.2.5, Oracle

Desc:   What is the optimal block size to use on reads with Oracle on
        raw logical volumes?

Action: This information has been taken from several databases:

        The LVM and disk device drivers make requests on 4kb boundaries.
        Data can be read from the disk more than 4kb at one time.  If
        less than 4096 bytes on a read is requested, the disk device
        driver will still retrieve 4kb, but will only hand the
        application the number of bytes requested.

        The controller on the disk reads and writes in increments of
        512 bytes.  The platter is formatted in 512 byte blocks.  When
        the disk device driver makes a request for some number of 4kb
        blocks, the disk controller does reads in blocks of 512 bytes,
        and returns them in increments of 4kb.

        Thus, the best block size to use for the reads from Oracle would
        be some multiple of 4096.

Next:   Close.



Support Line: LVM block size and disk reads for raw devices ITEM: X0137L

Dated: August 1995 Category: N/A

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